1. Field of the Invention
The present invention relates to the manufacture of magnetic structures and electric reactive components, and more specifically to an inductor formed on a printed circuit board.
The invention also falls within the field of switching voltage regulators and electronic power supplies, which convert energy from one level to another. These devices have been common in all electronic systems. More specifically, the invention falls into the class of voltage regulators referred to as buck and boost converters, which convert a voltage to a higher or lower voltage. The present invention further relates to passive components structures embedded on a printed circuit board for use in power conversion circuits and techniques.
2. Brief Description of Related Art
Switching power converters are common systems which typically have an input terminal for receiving an input voltage, and an output terminal which supplies current to a load. The output terminal provides a substantially fixed voltage independent of the magnitude of the input voltage or the current provided to a load. These components typically use combinations of switches, inductors, transformers and capacitors to implement highly efficient transformation of DC and AC power.
The magnetic elements, inductors and transformers, are typically built as discrete components using multiple turns of wire around ferromagnetic cores. The use of ferromagnetic cores provides both higher inductance values in a given volume and suppression of stray magnetic fields.
There is continual demand for improved efficiency in the power conversion components. Many switching voltage regulators have been replacing the more common linear regulators, however in some specific consumer applications the use of switching power converters has not been possible for several reasons, the most common being the inductor's cost and in some cases the critical height requirements for the components on the circuit board.
The size of the inductive element and its cost increase with the inductance of the components and its current carrying capability. In order to minimize both the cost and the height of the inductor, it would be reasonable to use a lower value inductor. In order to use small inductance inductors, the switching frequency must increase. Increasing the switching frequency causes switching losses in the solid-state power switches and their associated drivers, but more importantly the magnetic losses in the inductor become predominant, mainly due to the magnetic hysteresis and to the Eddy currents in the ferromagnetic cores. In particular this second contribution to the magnetic losses is increasing with the square of the switching frequency. The Eddy currents are generated in any electrical conductive element that is close enough to the inductor to be crossed by the magnetic field lines. The Eddy currents reveal themselves in an equivalent way to more traditional resistive losses.
At very high frequencies, it is a common RF (Radio-Frequency) technique to utilize the inductance of a metal trace of an integrated circuit or of a printed wiring trace as a known inductive element to form filters, antennas and matching networks. Although the inductance values thus achieved are generally quite low (tens of nano Henrys), this is a practical technique for many RF applications. There are well known problems with this technique, as the resulting inductors have generally lower “Q” than can be generated otherwise, and adjacent inductors will tend to “couple” in manners that can be difficult to manage.
Based upon a long history of the use of printed wiring inductors in RF applications and a variety of attempts to integrate inductor and transformer windings onto the PCB, it is conventional wisdom that air core inductors formed by printed wiring boards are impractical for power conversion applications for several reasons:                a) Inductance values too low,        b) Inductor Q is poor,        c) Inductor consumes large board space,        d) Inductor creates large undesired magnetic fields.        
While these objections were at one time quite valid, the subject invention makes it possible to build air core magnetic structures on the printed wiring board that can provide adequate performance also for switching power conversion components. The following issues mitigate the known problems.
In recent decades, particularly following the introduction of the power MOSFET, switching frequencies for switching power supplies have migrated from 20 kHz to well over 1 MHz. Since the output power of a switching converter is proportional to the switching frequency and to the inductance value, the reduction of the time period between switching cycles has allowed the use of smaller inductance inductors. In addition a higher switching frequency is naturally producing a lower output voltage ripple, typically requiring a smaller filter output capacitor.
A limiting factor in many high frequency switching power circuits is the power dissipation in the magnetic structure due to the lossy nature of ferromagnetic material at high frequencies. The magnetic hysteresis intrinsic of any ferromagnetic material causes a dissipation that is typically increasing linearly with the switching frequency. In addition Eddy currents in the core, increase quadraticly with the frequency and they contribute to the total magnetic loss. These limitations can be overcome with an air core inductor, in fact the lower magnetic permeability of air and, more importantly, its inherent linearity eliminates totally the magnetic losses.
The speed of the electronic circuitry on integrated circuits and the switching speed of power MOSFET devices pose no present barrier to raising switching frequencies even higher. The higher the switching frequency, the lower the required inductance in any magnetic element. A lower inductance associated with air core inductors represent a high efficiency solution, provided that means for reducing the radiated energy are implemented.
The Q of printed circuit inductors is limited by the resistance of the printed circuit trace, which has much smaller cross-sectional area than the typical round copper wire used to manufacture inductors or transformers. However, at high frequencies, the effective resistance of the winding is often limited by the “skin effect”, wherein most of the current flows only in the outermost region of the conductor. The large cross-sectional perimeter of printed traces can be advantageous at high frequencies. The resistive loss of the printed wiring solution may still be greater than that of a conventional magnetic component and nevertheless have lower overall loss due to the lack of a lossy ferromagnetic core.
The board space consumed by printed circuit inductors has a cost, but on multi-layer boards, a conductive winding made on an inner layer of the board uses no surface area and adds no height constraint. In many modern systems, board space may not be so critical as the height of components on the board, which often have stringent height requirements due to small mechanical packages. The decreasing inductance value of the magnetic components as switching frequencies increase also contributes to the shrinking of required board space.
The use of multiple anti-phased windings, as disclosed in the present invention, makes a considerable impact on reducing both far-field and near-field Electro Magnetic Interferences (EMI) concerns. The coupling of the magnetic field of a printed wiring inductor to nearby circuitry due to the stray magnetic fields can be minimized by the subject invention.
The conventional means of creating an inductor in an integrated circuit or in a printed wiring board is the spiral inductor, as shown in FIG. 1A. The spiral inductor can be characterized by its outer diameter, its inner diameter, the number of turns and the width (and space) of the copper traces. Because of the spiral nature of the structure, outer windings have a larger diameter than inner windings, such that the nominal inductance of each winding varies. FIG. 1B shows the inductor L1 with its associated magnetic lines, when current is flowing in the inductor.
A well-established principle in constructing practical inductors is the mutual inductance of windings which will produce a common magnetic flux. When multiple windings, which are not coupled, are placed in series, the total inductance is the sum of the individual inductances. When n windings that are well coupled are placed in series, the inductance increases by a factor of n*n, that is in a quadratic way. It is also possible, by reversing the polarity of coupled windings, to reduce the effective inductance to less than the sum of the individual windings.
In the spiral inductor, adjacent windings can be well coupled, but because each turn has progressively changing inductance, the coupling of one winding to the next cannot approach unity. If a second spiral inductor with similar diameter, etc. is stacked above or below the first in very close proximity, the coupling between the two spiral inductors can be very close to unity.
An unfortunate manner of coupling however is the coupling to any closed conductive path that surrounds the spiral. Even if coupling is significantly less than unity, the coupling makes any such path look much like a poorly coupled secondary on a transformer where the primary is the spiral inductor. In the case of modern printed wiring boards, this means that any ground plane that might encircle the inductor would be a shorted turn on such a transformer, which will reflect back to lower the inductance and to increase losses significantly as well as inducing a circulating current in the ground plane (Eddy currents) and an associated induced voltage between differing points on that ground plane.
The current flowing in a spiral inductor generates a magnetic field whose magnetic lines are perpendicular with the spiral plane. The magnetic field lines are always closed, and their path is uniformly distributed around the spiral with intensity decreasing with the square of the distance from the inductor. This stray magnetic field spreading around the inductor may cause undesired effects. A means of containing the magnetic field in order to minimize the effects from radiated energy is disclosed in this invention.
The use of printed wiring inductors in power conversion applications has been limited primarily to the use of windings on a printed circuit board being used in conjunction with a ferrite core to produce a low profile inductor or transformer. A prior art example of a low profile transformer is disclosed in Williams (U.S. Pat. No. 4,873,757).
A further prior art example of a low profile inductor using printed wiring board in conjunction with ferromagnetic cores is disclosed in Godek et al. (U.S. Pat. No. 5,565,837). Such assemblies use the inherent ease of manufacture of three-dimensional wiring within the circuit board to create extremely consistent windings eliminating the need for mechanical bobbins, windings, and the interconnection of the windings to the circuitry on the printed wiring board.
Another prior art application of printed wiring magnetic structures for power conversion has been the use of coupled windings on the printed wiring board as a pulse transformer (as disclosed in IEEE Transactions on Power Electronics, Vol. 14, NO. 3, May 1999 “Coreless Printed Circuit Board (PCB) Transformers with Multiple Secondary Windings for Complementary Gate Drive Circuits” by S. C. Tang et al.). This application used the inductive element of the transformer as a signal transmission element rather than as a means of processing power, and as mentioned, illustrated some of the potential problems with using magnetic structures composed of printed windings.
A further prior art application of printed board spiral inductor is disclosed in Iwanami (U.S. Pat. No. 6,384,706). This multi-layer printed board features plural spiral-shaped interconnected structures in conjunction with insulative magnetic layers between the structures to maximize the total inductance for use as de-coupling (filter) inductor of high frequency currents from the power supplies to the integrated circuits.
Another prior art application of multi-layered printed circuit board inductor or transformer is disclosed in Folker et al. (U.S. Pat. No. 5,777,539). This inductor or transformer uses a stack of conductive layers to form several turns with a ferrite core that passes through a hole in the printed circuit board within the conductors.
A further prior art application of printed wiring board with integrated coil inductor is disclosed in Tohya et al. (U.S. Pat. No. 5,978,231). A power conductive layer and a ground conductive layer are partially cut to form conductors that are connected through via holes in order to form a spiral inductor. An electric insulating ferromagnetic layer is also added to increase the total inductance.
A further prior art application of printed circuit board inductor is disclosed in Eberhardt (U.S. Pat. No. 5,461,353). A spiral inductor is formed connecting conductive paths on two intermediate separate layers shielding this inductor with a top layer and a lower layer to reduce the magnetic stray field.
A prior art application of air core inductor for power conversion is disclosed in IEEE Applied Power Electronics Conference March 1999 “Design of Microfabricated Inductors for Microprocessor Power Delivery” by G. J. Mehas et al. In this paper the use of an air core inductor, as a shorted coaxial line to reduce the loss and EMI in nearby conductors, was considered but not deemed practical due to the low power density of the coaxial cable.
For high performance power conversion applications multi-phase converters are very common. There are several reasons to justify the multi-phase converters approach, in particular for step-down converters, and they are:                a) simplicity of design and implementation because the load current is actually divided among the multiple phases,        b) overall space consumed by the magnetic elements,        c) reduced output voltage ripple, and        d) efficiency.        
In particular, since the magnetic losses of conventional ferromagnetic core inductors are limiting the switching frequency of the converters, the use of multiple phase converters to achieve low output voltage ripple is the conventional approach. An inexpensive means of eliminating the magnetic losses is disclosed in this invention. That approach could lead to the implementation of higher frequency single-phase converters for high current and high performance applications reducing cost and complexity.
Accordingly, what is needed is a low cost, low EMI inductor for power conversion circuits that combines the advantages of high efficiency (allowing high frequency switching without adding undesired magnetic losses) and minimum board height requirements (not impacting the height of the final application circuit board). This would allow operation for the conventional and higher frequency step-up and step-down switching voltage converters minimizing the size and cost of output capacitors and reducing the output voltage ripple.